The present invention relates to a new and improved construction of an ionization fire alarm or detector.
In its more specific aspects the invention relates to a new and improved ionization fire alarm or detector which is of the type which contains a radioactive substance located in an ionization chamber accessible to the ambient atmosphere and two electrodes separated by an insulating member. An electric circuit serves for signalling and triggering an alarm.
In ionization fire alarms or detectors as known, for example, from German Pat. No. 2,130,889 the air present in the ionization chamber which is accessible for the ambient or external atmosphere becomes ionized due to the radioactive substance or specimen located therein. Due to the d.c.-voltage applied to the two electrodes of the ionization chamber an ionic current flows between the electrodes. In the event that smoke, aerosols formed during a fire or other particles enter the ionization chamber through apertures or openings of the chamber then the electric current will change. The electric circuit evaluates the current change such that at a certain reduction in conductivity in the ionization chamber an alarm signal is transmitted to a central signal station via electric lines. In known circuits of such kind the ionization chamber is connected in series with a resistor element which, for example, constitutes a reference ionization chamber which is either very nearly hermetically sealed or insensitive to aerosols formed during a combustion process. The potential difference between the two chambers is determined using a high-ohm or high-impedance amplifier like, for example, a field-effect transistor. Another possibility resides in periodically scanning the charge on the electrodes in the ionization chamber.
Since the ionization chambers generally used have a resistance greater than 10.sup.10 .OMEGA., and thus, the electric circuit must have an input resistance which is substantially higher, the ionization fire alarms or detectors are highly sensitive to any contamination which reduces the electrical resistance i.e., of the insulating member or of the insulating path between the electrodes of the ionization chamber. In the same fashion as aerosols generated by a fire or combustion process other particles emanating from the environment of the detector like, for example, dust are transported into the ionization chamber and will be deposited therein, whereby the electrical resistance of the insulating path will decrease. Consequently, frequent maintenance of fire alarm systems and cleaning of the ionization fire alarms or detectors is required.
According to the aforementioned German Pat. No. 2,130,889 the problem of maintaining the electrical resistance is solved by covering the insulating path or span in the interior of the hood, which serves as the outer or external electrode and contains apertures for the entry of environmental air, by a labyrinth formed by the same highly insulating synthetic material. Thus, the creepage path between the central-electrode and the counterelectrode is protected from contamination, and the creepage path exposed to contamination is enlarged by more than a factor of 4 due to the annular or ring-shaped webs of the labyrinth. It has thus become possible to considerably prolong the period of time until the fire alarm or detector becomes ineffective, i.e. the service or maintenance intervals can be increased. However, synthetic materials, i.e. plastics, are subject to natural aging which may become accelerated due to the action of oxygen in the air or, partially also due to ozone, or due to the action of aggressive ingredients or constituents contained in the environmental air or in the cleaning agents used for servicing the ionization fire alarm or detector. While such corrosive substances are present in standard environmental air only in extremely low concentrations, nonetheless the concentrations thereof may assume considerable values in specific environments. Finally, the duration of the action of the different substances cannot be neglected as well as the fact that the air is ionized by the radioactive source present in the ionization fire alarm, whereby ozone and other compounds or substances capable of attacking the materials of which the ionization fire alarm is composed are formed in the very interior thereof. Since the atmosphere may penetrate between the labyrinth and the insulating path the problem of aging of the insulating path persists.
Cleaning of the ionization fire alarms or detectors during servicing the same did not heretofore present a problem, however, repeated cleanings have always been problematic with respect to maintaining the high surface insulating values of about 10.sup.10 .OMEGA. over longer time spans. The search for materials having sufficiently high resistance towards environmental effects caused by, for example, solvent vapors, insecticides and so forth, did not render a satisfactory result, since it was impossible to find a plastic or synthetic material having optimum properties in respect of all environmental effects.